To open the driver or passenger side of a vehicle while moving whith the intent to hit some unsuspecting pedestrian with it.

Pete: Hey ross, open your door and hit that bitch Cole Knowels with it.
Ross: Dumbass, you know my door dont open on the drivers side! I'll go around the block so you can hit him with your door.
Pete: Alright, I'll door that little fags' ass

"Dooring" or "To Door" is a slang term for when someone is so jealous of a nice or expensive car, that they park their old wreck of a car next to it, and proceed to bash the old car's door into the new one, causing paint damage and a dent.